Monkey B virus (BV) is a normal pathogen of macaque monkeys that is closely related to human herpes simplex virus (HSV). Although BV is not a serious infection in macaques, when transmitted to humans it readily invades the central nervous system and, left untreated, has a fatality rate of approximately 80%. Due to the severity of BV infections, this virus has been included on the CDC Select Agent list as a potential bioterrorism agent. Chemotherapeutic treatment is the only option for controlling human BV infections, yet little is know about the efficacy of various drugs against BV. Current treatment for BV infections essentially utilizes drugs known to be safe and effective against HSV, the assumption being that they are also effective against BV since the viruses are related. We recently tested the efficacy of a number of nucleoside analogs against BV. In the course of these studies, two naturally occurring drug-resistant variants (PCVr & GCVr) were isolated. In HSV, all mutations conferring resistance to acyclo-nucleosides occur in the viral thymidine kinase (TK) and/or DNA polymerase genes. Sequencing of the TK and DNA pol genes of these two BV mutants revealed that they were identical to the sequence of the parental wild-type virus genes. Thus, some other virus gene(s) appears to be involved in determining the drug resistance of these BV isolates. This application will identify the viral gene(s) that are responsible for the drug resistance of these BV isolates. Using a co-transfection procedure, we will construct recombinant viruses to locate the region of the viral genome containing the drug resistance locus. Once this region has been localized, DNA sequencing will identify the precise sequence difference that defines drug resistance. Knowing the gene(s) involved in drug resistance of BV will allow some predictions to be made as to the mechanism involved and open the door to additional studies to test these hypotheses. The knowledge gained may also eventually lead to a better understanding of the comparative resistance of BV to drugs effective against HSV and to the development of more effective drugs for treatment of BV infections.